Increasing the filler content of paper has been driven by efforts to reduce papermaking cost, improve the water removal rate during papermaking, and improve optical properties and printability of the paper product. There are, however, limits to the amount of filler that can be substituted for papermaking fiber. At high filler contents, paper can suffer losses in strength, stiffness, bulk, and sizing. All commonly used unmodified fillers (e.g., clay, titanium dioxide, and calcium carbonate) are known to have detrimental effects on strength and sizing. Increasing the concentration of filler in conventional papermaking furnishes results in increased size demand to maintain the desired degree of water repellency in the finished paper. In particular, poor sizing efficiency and loss of water repellency over time are problems associated with the use of alkyl ketene dimer (AKD) and alkenyl succinic anhydride (ASA) sizing agents and precipitated calcium carbonate (PCC), particularly in highly filled alkaline papers. In addition, strength properties decrease as filler levels increase, impacting negatively on converting operations and end use functionality. Thus, in circumstances where increasing filler content would be advantageous, associated sizing problems have occurred affecting paper quality and machine performance.
Traditionally, starch has been used in wet-end papermaking applications because of its relatively low price and its ability to improve paper strength. It is believed that starch adsorbs to wood fibers during wet-end processing and enhances bonding through the formation of hydrogen bonds. The improvements in dry strength may also due to increases in shear strength per unit bonded area. Although starch can improve the strength of paper, high starch addition in wet-end papermaking process can also cause problems, such as, poor retention of raw starch in a pulp furnish. Cationic groups are usually introduced to enhance the adsorption of starch to anionically charged wood fiber surfaces. However, such chemical modification of starch significantly increases the cost of papermaking. Moreover, although the adsorption of starch can be improved by introducing cationic groups, the retention of starch on wood fibers may still be a problem when anionic trash in the wet-end furnish is high, e.g., when ground wood pulps and recycled fibers are used.
Although many approaches have been studied for improving the paper strength at high filler contents, such as, preflocculation, lumen loading, chemically treated filler with polymer, and filler-wood fine composites, very few have delivered a practical solution to paper manufacturers.